How to make a Textbox accept only the letters S and N?

Question

I’m studying about TextBox in C# and I have no idea how to do this function.

Answer 1

What happens immediately is to intercept the pressed key, using the event KeyDown, and check which was.
If the key corresponds to S or N the implementation must return e.Handled = false;, otherwise return e.Handled = true;.

Unfortunately this is not enough, since the key space is not intercepted.
On the other hand there is the possibility that the text can be entered via Paste.

Thus, to deal with all possibilities, you must treat the following events:

• Previewtextinput
• Previewkeydown
• Paste

and restrict the number of accepted characters to 1.

The "best" way for the implementation is to write a custom Textbox that you can use anywhere.

public class S_N_TextBox : TextBox
{

    public S_N_TextBox()
    {
        DataObject.AddPastingHandler(this, PastingHandler);
        MaxLength = 1;
    }

    private void PastingHandler(object sender, DataObjectPastingEventArgs e)
    {
        var pasteText = e.DataObject.GetData(typeof(string)) as string;
        //Garante que o texto inserido via paste é válido
        if (!IsValid(pasteText))
        {
            e.CancelCommand();
        }
    }

    protected override void OnPreviewTextInput(TextCompositionEventArgs e)
    {
        if (IsValid(e.Text))
        {
            //A documentação recomenda que o método base seja chamado.
            base.OnPreviewTextInput(e);
            return;
        }
        e.Handled = true;

    }

    protected override void OnPreviewKeyDown(KeyEventArgs e)
    {
        //Não permite o uso do espaço
        e.Handled = e.Key == Key.Space;
    }

    //Determina que texto pode ser aceite
    protected virtual bool IsValid(string input) => input == "S" || input == "N";
}

This class can be used as a basis for other restrictions you want a Textblox to have. You only have to change the method IsValid().

Answer 2

In the archive .xaml of his Window put the following code. What matters here is the event KeyDown="txtTest_KeyDown":

<TextBox x:Name="txtTest" KeyDown="txtTest_KeyDown" HorizontalAlignment="Left" Height="23" Margin="239,148,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="282"/>

Now, in the .cs of his Window, create the method txtTest_KeyDown as follows:

private void txtTest_KeyDown(object sender, KeyEventArgs e)
{
    if(e.Key == Key.S || e.Key == Key.N)
    {
        e.Handled = false;
    }
    else
    {
        e.Handled = true;
    }
}